1. Field of the Invention
The present invention relates in general to a ceramic substrate having a lower electrode for forming a thin-film condenser on the surface of the substrate and more particularly to a ceramic substrate having a lower electrode which is constructed so as to prevent an insulation defect, short defect, etc. of a condenser.
2. Description of the Prior Art
Due to recent development of the integrated circuit technology, the operation frequency of the circuit has become considerably higher for attaining a high-speed operation. With this high operation frequency, it has been practiced to install or form a bypass condenser on a ceramic substrate with a view to preventing an erroneous operation of the integrated circuit.
Because the condenser is required to be compact in size in itself notwithstanding it is also required to have a relatively large capacity and because of the properties of the condenser, it has been considered to form the condenser on a surface portion of the ceramic substrate adjacent the integrated circuit.
In this instance, in order to attain a large electrostatic capacity within a limited area, it is necessitated to reduce the thickness of the dielectric layer, so it has been practiced to form the dielectric layer by a thin-film forming technique such as spattering, CVD (chemical-vapor deposition), sol-gel method, etc.
However, in case of forming a dielectric layer which is so thin as to be several microns or less thick, the surface conditions of the substrate and the lower electrode become important.
This is because, when the surface of the substrate is rough, it is impossible to form and laminate the lower electrode and the dielectric layer uniformly, resulting in a possibility that a short circuit due to contact between the upper and lower electrodes or breakdown at a low voltage is caused to adversely affect the properties of the condenser.
Though the rough surface of the substrate can be improved to some extent by grinding or polishing, grinding for attaining sufficient smoothness requires too much man-hour and therefore a high cost. Furthermore, in case a ceramic substrate is used, it is inevitable that the surface of the substrate are formed with recessed portions due to pores of the ceramic substrate, and furthermore a problem may be caused by the irregularity or unevenness of the surface of the substrate due to flaws or scratches at the time of grinding.
Accordingly, in case of forming a thin-film condenser, it has been practiced to use a Si-wafer as disclosed in Japanese patent publication (after examination) No. 60-55975 or to use a ceramic substrate which is glazed to have a smoothed surface. However, the Si-wafer is encountered by a difficulty in its installation on an integrated circuit package, and the glazed ceramic substrate is expensive.
While it has further been proposed to form a thin-film condenser on a ceramic substrate which is not glazed as disclosed in Japanese Patent after-examination publication No. 60-55975 and Japanese Utility Model provisional publication No. 5-13066, the condenser according to the former publication has a problem that it requires a difficult manufacturing process, thus increasing the cost, and furthermore it is poor in the heat-resisting property, whereas the latter publication is incapable of offering a fundamental solution to the above described problem since its lower electrode is formed by vapor deposition.
Further, by strictly controlling the employed method of producing a ceramic substrate so that the ceramic substrate which is extremely reduced in the number and size of pores is obtained and by paying sufficient attention to the grinding so that the surface of the ceramic substrate is not formed with any flaw or scratch at the mime of grinding, the surface of the ceramic substrate having a small irregularity can be obtained, thus making it possible to solve or at least mitigate the above described problem. However, the manufacture or production of such a ceramic substrate is difficult, thus resulting in a high manufacturing cost, and furthermore the properties of such a ceramic substrate are different from those of the ceramic substrate having pores in the surface thereof, thus causing a design inconvenience.